With Worldwide Interoperability for Microwave Access (WiMAX), Long Term Evolution (LTE), which is a standard for the 3.9G mobile communication system, or the like, “orthogonal frequency division multiplexing” is used as a basic transmission system. With the “orthogonal frequency division multiplexing (hereafter, OFDM)”, what are called the sub-carriers are transmitted in parallel by using multiple low-speed circuits combined so that it is possible to perform high-speed data transmission that has resistance to intersymbol interference.
Because OFDM has a disadvantage in that the ratio (hereafter, PAPR) between the peak power, which is the value with the maximum amplitude in the signal waveform, and the average power is large, a power amplifier included in a communication device that uses OFDM is requested to have high linearity. FIG. 13 is a graph that illustrates an OFDM signal waveform, which is plotted along the time axis. FIG. 14 is a graph that illustrates the input/output characteristics of a power amplifier. As illustrated in FIG. 13, the PAPR is large in the OFDM signal waveform, which is plotted along the time axis. As illustrated in FIG. 14, the power amplifier has the input/output characteristics such that, if the input power becomes large, the output power is saturated. For example, if a signal with a large amount of power, such as the peak power, is input to the power amplifier, the signal is not correctly output, which results in the distortion of the signal waveform.
For this reason, there is a method of operating a power amplifier within the region that is obtained by subtracting a back-off value from the output power (hereafter, saturation power) during the saturation. A modulation accuracy (hereafter, Error Vector Magnitude (EVM)) is usually decreased if a signal waveform is distorted. Therefore, it is desired to ensure a sufficiently large back-off value; however, a power amplifier with a large saturation power has to be used, which significantly reduces the power added efficiency. As a result, conventional methods of reducing the back-off value to an appropriate value are predominant.    Patent Literature 1: Japanese Laid-open Patent Publication No. 2001-251262    Patent Literature 2: Japanese Laid-open Patent Publication No. 2001-068940
In the above-described conventional technology, however, there is also a problem in that the suppression of the PAPR of an input signal causes a decrease in the EVM. This problem also occurs, not only when the OFDM is used, but also when the Orthogonal Frequency Division Multiple Access (hereafter, OFDMA) is used as a transmission system.